C# Discussion Boards - CodeProject

Re: code dataGridView1[e.ColumnIndex, e.RowIndex].Value.ToString() doesnt work

2-May-15 7:18

So use the debugger and find out which bit is null.
Then you can look at why!

But I can't do that for you - I have no access to your computer...

Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...

Henrik Jonsson2-May-15 7:05

Henrik Jonsson

2-May-15 7:05

Hi,

when you get "Object reference not set to an instance of an object" exceptions some of the properties you invoke have returned null, in this case most likely Value for an empty grid cell. Check the Value for null before using it:

var gridValue = dataGridView1[e.ColumnIndex, e.RowIndex].Value;
if( gridValue != null )
{
    string gridStringValue = gridValue.ToString();
...
}

Re: code dataGridView1[e.ColumnIndex, e.RowIndex].Value.ToString() doesnt work

V.4-May-15 22:58

4-May-15 22:58

string cellval = "";
if(dataGridView1[e.ColumnIndex, e.RowIndex].Value != null){
  cellval = dataGridView1[e.ColumnIndex, e.RowIndex].Value.ToString();
}

I usually use Convert.ToString(...); in such cases, because it will convert a null to "" automatically.
(not useful in all cases)

V.

(MQOTD rules and previous solutions)

How can I write string to a memory mapped file?

Member 116587422-May-15 0:52

Re: How can I write string to a memory mapped file?

2-May-15 0:52

I want to write a string to a memory mapped file. I have used Marshalling to achieve desired size of string. I am getting an exception when I try to write my structure which contains a string and an int. If I remove the string, The code runs successfully.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;
using System.Runtime.InteropServices;
using System.IO.MemoryMappedFiles;

namespace ConsoleApplication25
{
    class Program
    {
        static void Main(string[] args)
        {
            MemoryCreate<MemWriter> SharedData = new MemoryCreate<MemWriter>();
            MemWriter MemWriterObj = new MemWriter();
            if (!SharedData.open()) return;
            MemWriterObj.f=10;
            MemWriterObj.mystring = "Test";
            SharedData.Writer = MemWriterObj;
            Console.ReadLine();
            SharedData.close();
        }
    }
    class MemoryCreate<T> where T:struct
    {
        MemoryMappedFile mmf;
        MemoryMappedViewAccessor accessor;
       public bool open()
        {
           try
           {
               mmf = MemoryMappedFile.CreateNew("Test", 100);
               accessor = mmf.CreateViewAccessor(0, 0);
               return true;
           }
           catch(Exception e)
           {
               Console.WriteLine(e.Message);
               return false;
           }
       }
           public void close()
           {
               accessor.Dispose();
               mmf.Dispose();
           }
        public T Writer
           {
               set
               {
                   try
                   {
                       accessor.Write<T>(0, ref value);
                   }
                   catch(Exception e)
                   {
                       Console.WriteLine(e.Message);
                   }
               }
           }
        }
    [StructLayout(LayoutKind.Sequential,Pack=1,CharSet=CharSet.Ansi)]
    struct MemWriter
    {
        public int f;
        [MarshalAs(UnmanagedType.ByValTStr,SizeConst=16)] public string mystring;
    }
}

[edit]Code block added - OriginalGriff[/edit]

modified 2-May-15 7:15am.

Richard MacCutchan2-May-15 3:33

Re: How can I write string to a memory mapped file?

2-May-15 3:33

What exception?

Member 116587422-May-15 4:17

Re: How can I write string to a memory mapped file?

2-May-15 4:17

The exception message is "The specified Type must be a struct containing no references. Parameter name: type"

Richard MacCutchan2-May-15 4:23

Re: How can I write string to a memory mapped file?

2-May-15 4:23

Assuming the exception occurs where I guess (since you have not bothered to tell us), I suspect it may be because mystring is a reference type.

Member 116587422-May-15 4:34

Re: How can I write string to a memory mapped file?

2-May-15 4:34

Oh I am sorry.
The exception is raised when I try to write the struct in memory mapped file i.e. this line - SharedData.Writer = MemWriterObj;
I have marshalled mystring hoping that it would solve the problem but it doesn't.
Any idea how I can make it work?

Richard MacCutchan2-May-15 4:56

Re: How can I write string to a memory mapped file?

2-May-15 4:56

Take a look at Programming Memory-Mapped Files with the .NET Framework[^].

Member 116587422-May-15 5:19

Re: How can I write string to a memory mapped file?

2-May-15 5:19

Thank you.
But I have already checked this, it shows how to write struct having int(which I am able to do successfully), I want to write a struct having string.

Richard MacCutchan2-May-15 5:28

Re: How can I write string to a memory mapped file?

2-May-15 5:28

Well, as explained, you can't; you can only write value types in a struct. If you want to write a string then you need to use one of the alternate methods, as shown in the article.

OriginalGriff2-May-15 4:30

Re: How can I write string to a memory mapped file?

2-May-15 4:30

Basically what the error is saying is "you can't do that at all".
A struct is a value type - it contains all it's information directly within the struct itself, and teh size is always fixed at compile time. "Basic types" such as int, float, and double are also value types.
A string is a reference type - the variable you declare is not the string itself but a reference to the memory that actually contains the string data - the reference itself is a fixed size, but teh data it refers to is not.

The error message is saying "you can only give me fixed size data" - a value type, not a reference type - and that it cannot contain any references.

So the struct you pass through can only contain basic types, and structs containing basic types - no reference types, which means no strings, no arrays, no Lists, nothing complex at all.

I suspect you are going to have to find a better solution to your problem, and drop the idea of memory mapped files altogether. What exactly are you trying to do that you think you need these?

Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...

Member 116587422-May-15 4:40

Re: How can I write string to a memory mapped file?

2-May-15 4:40

@OrigianlGriff...That is the reason I used MarshalAs(UnmanagedType.ByValTStr,SizeConst=16)] public string mystring;
Which gives my string a fixed size(16) if I am not wrong.
I am trying to create an application which writes to a memory mapped file("test"). There will be another application which reads from the same memory mapped file and performs the specified operation.

OriginalGriff2-May-15 4:54

Re: How can I write string to a memory mapped file?

2-May-15 4:54

Doesn't matter: it's still a reference type, and you can't use it!

Are you just trying to use the file to communicate between the two apps?

Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...

Member 116587422-May-15 5:11

Re: How can I write string to a memory mapped file?

2-May-15 5:11

Ok.
Is there no other way to do it?

Sascha Lefèvre2-May-15 5:25

Sascha Lefèvre

2-May-15 5:25

For writing data to an MMF that is more complex than a struct containing only value type members you would have to serialize that data into a byte array and then write that byte array to the MMF using MemoryMappedViewAccessor.WriteArray<T>[^]. In order to be able to read it back, you should also write the length of the byte-array to the MMF, probably directly before the start of the byte-array.

If the brain were so simple we could understand it, we would be so simple we couldn't. — Lyall Watson

Help me, I need a simple program examples.

Member 114460801-May-15 20:55

Member 11446080

1-May-15 20:55

I need a runtime is similar to the Visual Studio forms designer examples, I hope it's not a Windows form, but Windows Presentation Foundation, it has a few features I need:

1. to design window

2. in the design window, add widge

I am very grateful to you if you help me

Re: Help me, I need a simple program examples.

OriginalGriff1-May-15 21:16

Problem in killing both Application.exe and launched process Extract.exe from Application.exe in multiple instances environment

1-May-15 21:16

And?
What have you tried?
Where are you stuck?
What help do you need?

Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...

shivmymail1-May-15 19:02

shivmymail

1-May-15 19:02

CSS

Environment: - Windows XP/Windows7 32bit, 64 bit/Windows 2008 Server 64 bit/Windows 2012 Server 64 bit
.
Application Details: -
Application.exe [Visual Studio C# created]
Extract.exe [Visual Studio C# created]
.Net Frame Work: - 3.5 .Net Frame Work
.
Product Description: - We have a product Application.exe, which launch another process Extract.exe in command line for extracting image documents, after extraction; we store data into text file and read from Application.exe and store into data base.
Note: - Application.exe start from services.msc.
.
We run multiple instance of Application.exe after renaming it as Application-FirstInstance.exe, Application-SecondInstance.exe; each application exe launches Extract.exe. We create service for each Application-{Instance}.exe. Application works fine in multiple instances environment, but during run of each service; there are many Extract.exe and Application-{Instance}.exe visible in task manager.
.
Requirement Description: -
We want to kill related Extract.exe on stopping Application-{Instance}.exe from services, and it would be instant operation.
But we have to make sure that related Extract.exe is deleting, while stopping Application-{Instance}.exe from services. What will be right way to identifying particular Application-{Instance}.exe and related Extract.ext and killing operation?
.

Kindly assist us to find the resolution. Please let us know, if more information is required.
We will be really thankful.

Re: Problem in killing both Application.exe and launched process Extract.exe from Application.exe in multiple instances environment

OriginalGriff1-May-15 19:34

Re: Problem in killing both Application.exe and launched process Extract.exe from Application.exe in multiple instances environment

1-May-15 19:34

The best solution is that when an instance of Application.exe is killed, it should close its instance of Extract.exe - since it is the only program that knows for sure which one it is.

Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...

shivmymail1-May-15 19:40

shivmymail

1-May-15 19:40

Hi Sir,
That is right, but how will we know the reference of particular exe.

Re: Problem in killing both Application.exe and launched process Extract.exe from Application.exe in multiple instances environment

OriginalGriff1-May-15 19:44